"ou™';.May°?:i?!a'] COKBESPONDENCE. 259 



have both seen the lines of the 17th band [over 101,000] with it; no 

 more at present ; but this is an extraordinary feat for a ^th, and is more 

 than any other objective of that power that I know of can possibly be 

 stretched to do." 



Messrs. Nobert and Eulenstein both saw the lines of the 17th band 

 with Powell and Lealand's tV^I^ immersion, but curiously enough they 

 think they got the best effect with both objectives dry ! I am con- 

 vinced that the ^th will do best wet, and therefore that they did not 

 accomplish all that they might have done with it, but that by perse- 

 verance they could have resolved the 19th band. 



As only two years previously Nobert had never seen the lines of 

 his test-plate finer than the 14th band, and then doubted the visibility 

 of any finer lines, his having now seen the 17th band with the ToUes' 

 ^th is conclusive evidence of the merits of the instrument, and may 

 relieve Mr. Lobb's doubts, and Dr. Woodward's disbelief as to what 

 had been seen. 



On the value of Nobert's lines as a test, the President says, " the 

 visibility of the lines is a function of the breadth of the groove ploughed 

 in the glass, — the depth to which it is cut, — the section of the groove 

 itself, — and the direction and character of the illumination employed, 

 — all these variable conditions in some measure detract from the fixed 

 value of this test." 



Of course they do ; and the same variable conditions affect all 

 natural objects in the same manner, and in the case of the Diatomacefe 

 in a much greater degree. I do not know what Mr. Powell calls " Acus " 

 (there is no such genus in Pritchard), or what he has to prove that it 

 is a " sharper and safer test ;" but I can say confidently (and other 

 observers confirm me), after years of work on the diatoms, and on the 

 test-plates, no less than four of them, that the plates are more uniform, 

 more reliable, and hence " safer" than any diatom I have yet seen, for 

 testing different instruments, in different places. It needs no argu- 

 ment to show that if all observers could use the same one object, 

 whether natural or artificial, that such is the only absolute test. That 

 being of course impossible, I contend that the "test-plates" of Nobert 

 are more uniform, more reliable, and " safer " for comparison, than 

 any other object yet known to microscopists. 



Chables Stodder. 



To the Editor of the ' Monthly Microscojjical Journal.^ 



Boston, U.S.A., April 5, 1870. 

 Dear Sir, — I have read Mr. Stodder's very full and clear paper, 

 which will be forwarded by this mail for publication in yom- Journal, 

 on the subject of resolving the 19th band of the Nobert test-plate, and 

 fully agree with his statements, which, as nearly as I can remember, 

 are correct in detail and dates. 



I have not kept the close run of this discussion that Mr. Stodder 

 has, although personally interested, and was not present at the trial of 

 objectives before the Committee of the Mechanics' Association, but at 

 a meeting of the " Section of Microscopy " of the Boston Natural 



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